First Edition Dated September, 2000 46
Key
A
Part B
In the example of part B above, more than likely this part would be
manufactured by indexing to one of the sides of the part and then the four
holes drilled. However it must be designed (documented in product definition)
and measured as shown to meet customer requirements, as measurements
from the edge of the part to the holes are not important to the customer. This
is a case where the requirement to define datums according to how the part is
used overrides the requirement to define them as fabricated.
The requirement also refers to functional tests. Functional tests must be
directly related to the system inputs and to the other upstream and
downstream functional tests performed in the build tree. These relationships
must be documented.
Section 4.5.5 Engineering tolerances shall be allocated to all
levels of assemblies and detail parts to account for process
capabilities.
The goal of this requirement is to take manufacturing process capabilities into
account in order to achieve robust designs. Another popular industry concept
called Design for Manufacturing and Assembly (DFMA) has this concept at its
core. Under both concepts, designs that do not take process capabilities into
account can inadvertently end up with tolerances defined that are tighter than
the ability of the process. No amount of effort by the fabrication or assembly
mechanics can overcome the problem. Defects, assembly problems and
increased inspection will be unavoidable, and unit cost will end up higher than
estimated.
Requirement 4.5.3 helps ensure the design accounts for all stages of the
build process. In executing this requirement, a manufacturing plan is
generated at each level of build before the design can be made to match.
Requirement 4.5.2 can help one use the build tree and manufacturing plans
created by 4.5.3 to flow down the build and design requirements.
Requirement 4.5.5 then comes into play as personnel in each affected build
position review the requirements, associated tolerances and the ability to
